Plasma processing is widely used in the semiconductor industry for deposition, etching, resist removal, and related processing of semiconductor wafers and other substrates. Inductive plasma sources are often used for plasma processing to produce high density plasma and reactive species for processing wafers. For instance, inductive plasma sources can easily produce high density plasma using standard 13.56 MHz and lower frequency power generators.
A common element of any low pressure or vacuum plasma processing system is a vacuum seal that separates a low pressure plasma volume from the surrounding atmosphere. The integrity of the vacuum seal is extremely important for the plasma generation system as any gas leaks through the vacuum seal can change the chemical composition of the processing plasma. This can affect the process results or can even destroy the plasma.
Many plasma processing systems have multiple vacuum seals, some of which are in contact with very hot surfaces resulting from the plasma heat. This can lead to a short lifetime of the vacuum seals. While the cost of the vacuum seal itself is relatively high, the main cost of the failure of a vacuum seal is associated with process interruptions necessary to repair or replace the vacuum seal, which lowers yield. To avoid processing downtime, the lifetime of the vacuum seal should be longer than the period between scheduled plasma processing tool maintenance.
Vacuum seal failure can happen in many plasma sources independent of the specific mechanism of plasma generation. The power of the plasma source and the process time at which the vacuum seal fails can depend on the type of source, the material of the sidewalls (e.g. quartz) and other details. However, at some power a heat load from plasma for any kind of source becomes very high which can result in vacuum seal failure. The plasma-processing tool can be designed such that the vacuum seal is located further away from the plasma, thus reducing the heat load. This technique, however, only partially addresses the problem because the sidewall adjacent to the seal area will slowly warm up due to heat conductivity. In addition, this technique is difficult to apply to existing plasma sources because any significant change in the design will require requalification of the tool.
Thus, a need exists for a system and method to protect vacuum seals in plasma processing equipment such that the life of the vacuum seal is extended. A system and method that can be retrofitted or applied to existing plasma source designs would be particularly useful.